The present invention relates to a process for manufacturing large-size porous shaped bodies of low density by swelling clay masses by supplying heat thereto.
More particularly, the present invention deals with a process for the thermal, chemical and mechanical treatment of a clay-mineral mass in which a ceramification and reduction of the density of the mass is accomplished by swelling, in order to manufacture large-size cellular ceramic structural elements, e.g. multi-story high wall elements having a low weight. Basalts, pearlites, shales and clays can principally be considered as appropriate for the mass.
It has been proposed to insert preformed clay masses in the final outer dimensions of the shaped body and to swell the body exclusively with the aid of a device from below without any further outside assistance from any device, whereby the clay mass is intended to swell into the inner free spaces of the clay mass. Thus, it is proposed in German Patent 22 16 463 that clay masses preformed in an ingot mold be swollen on furnace carriages in a double-tunnel furnace. In DE-OS 36 35 672 and DE-OS 36 21 845 Al it has been proposed that shaped bodies with channels drawn through them be baked in a rapid-burning bogie hearth furnace and that the channels of the shaped body be filled by swelling. If a clay mass preformed according to the dimensions of the final shaped body is used, the heating of the clay mass to the temperature which is necessary for the swelling takes a very long time because of the large dimensions of the body, which means considerable expenditure on energy and on the apparatus. Moreover, in the case of the known processes, adquate accuracy of shape of the softening clay mass cannot be guaranteed on account of insufficient support of the swelling clay mass by the apparatus at the top and at the sides.
In addition, DE-AS 1 942 524 discloses a process for the manufacture of thermally foamed shaped parts, in which the material is initially preformed into shaped bodies such as plates, slabs and the like, and is then conveyed through a continuous furnace with the aid of an air bed and is simultaneously foamed. Air bed conveyance is, however, very costly as to energy and cost of the apparatus, as gas flows having high temperature, high pressure and a defined composition have to be created constantly.
Processes are known in which pre-swollen granules are intended to be molded into a shaped body without simultaneous heating. Thus, a process for manufacturing building blocks has been proposed (DE-AS 11 81 611), in which clay converted into particles is heated, and the particles which are treated in this way are pressed into shaped bodies. According to U.S. Pat. No. 3,274,309 and DE-AS 1 151 460, the shaping likewise occurs by adequately heated granules being compressed in a single mold by a stamp without any further supply of heat. The manufacture of a slab in an extrusion mold is proposed in DE-AS 23 14 297. It is also known for the material to be allowed to be pressed at the foot of a vertical column of material by the dead weight of the charged column (U.S. Pat. No. 1,892,583), and to be rolled out into a continuous slab by press rolls.
In all known processes the shaping is dictated by the pressing operation, and, because there is no further heat supply to the substance, is associated with unavoidable impairment of the inner structure and with irregular compression of the shaped body.
Processes have been proposed in which the shaped body is formed by the charge material being applied in layers, sintered in layers by direct action of heat on the top layer in each case, and swollen if necessary. Here, the particles drop (DE 21 24 146 C2) onto tunnel furnace carriages arranged one behind the other and touching each other, or onto a conveyor belt, and are collected, with the thickness of the layer which is produced being able to be adjusted according to the running speed of the conveyor belt. As the surfaces of the particles are sticky when they strike the conveyor belt, they become stuck or fused together (DE-AS 14 71 408). Because the bottommost layer is subject to a very much longer heat treatment time than the topmost layer, considerable irregularities in the body have to be reckoned with in this process, especially if the swelling occurs simultaneously with the shaping, and with too large an apparatus, resulting from too small a heat transfer surface in relation to the quantity of the clay mass being treated.
Processes are known in which the shaping occurs with simultaneous heating of the mold blank and in which a free foaming of the charge is carried out. Here an increase in the external volume of the charge, a closure of the interstitial volume in the charge and sintering of the partial masses of which the charge is composed, are accomplished simultaneously. It has been proposed to heat granules poured into single molds by means of heat supply across the mold walls without any controlled introduction of gases, and to sinter them into a body (DE-PS 22 16 463, DE-OS 2 147 645). Disadvantageously long treatment times for the substance in the mold region are necessary when heating the material charge merely by supplying heat from outside across the boundary surfaces of the material charge, especially without the introduction of heating gases. As the heat supply can occur only very slowly in this case, when the shaping occurs by sintering and swelling of the particles, the number of usable naturally swelling raw materials is small, as with slow heating only a few raw materials swell suitably.
According to DE-AS 26 04 793, an attempt is made to overcome the loss of swelling capacity through too slow a heating of the clay mass by the addition of certain foaming adjuvants which can be used with heating times of up to 180 minutes and heating speeds of 2.degree. C. per minute. The energy expenditure in the case of this process is very high on account of low charging density through using material of low density, large volume of the sintering mold and large necessary dimensions of the treatment apparatus, and also due to wear and high price of the single mold which have to be moved with the material. An increase in the speed of the heating results in a considerable difference in temperature between the edge temperature and the core temperature of the charge, whereby a severely delayed swelling or even no swelling at all occurs in the core zone of the charge, and thus, linked with this, an inhomogeneous pore size distribution. An important cause of the non-uniformity of the density distribution in cellular ceramic bodies, which occurs during swelling and sintering of the charge particles with an increase in the external dimensions of the charge and swelling of the interstitial volume, is the fact that the swelling begins with the supply of heat to the charge across the outer dimensions of the charge with no through-flow in the edge zones and corner zones of the charge, and the material which has swollen in the edge and corner zones expands into the free space above the charge. As material of low density has already swollen into there, the high density material which swells later on from the core zone can no longer expand, with the result that uneven density distribution occurs in the block.
According to DE-PS 29 41 370 C2, an attempt is made to compensate the unevenness of the swelling by heterogeneously compressing the pile of material before baking, whereby the edge regions are compressed more strongly than the core zone and the free space which is produced by the stronger compression is filled up with a further charge of material. In DE-OS-34 17 851 A1 it is proposed to produce highly porous ceramic shaped bodies with a uniform structure by baking the granulated and dried raw materials in capsule chambers which are sealed from the outset against the ambient atmosphere, at controlled excess pressure until they foam. According to DE-OS 35 38 783, it is proposed to obtain porous ceramic shaped bodies with a substantially uniform pore distribution resulting from uniform swelling up of the dried and preformed raw material, by using as a preformed raw material annular or hollow-cylindrical briquettes whose material volume takes up 40 to 60% of the internal space of the mold before the heating. The vigor of the necessary volume increase, in particular, which may lead to a fivefold increase in the volume of the clay mass, and consequently with the swelling of the charge, to a fivefold increase in the volume of the individual charge particles, in order to pass from the high density of the natural clay to that lower density of the block being manufactured, has, with free foaming, resulted in unavoidably irregular increases in volume due to mutual hindrance of the charge particles in thermal, mechanical, and in some cases, flow-mechanical respects and hence in a non-uniform density and shape structure of the product which is being manufactured.
In EP 87 114 811.0 (0 291 572 Al) it is proposed to deliver pellets to a circulating conveyor belt, to heat them by supplying heat without any controlled introduction of gases, and to foam them to fixed external dimensions by the counterpressure of a conveyor belt which holds them down.
The difficulties of the processes in which the sintering of the charge particles occurs with heating across the outer surface of the charge without gas flowing therethrough, and at the same time with the closure of the interstitial volume and with increase of the external volume, or even with a constant external volume of the charge, as in the case of the last-mentioned process, are altogether so fundamental on account of the long heat transmission paths, that overcoming them with the proposed additional measures is not possible. On the laboratory scale small blocks have been successfully manufactured by simultaneous swelling and sintering of the charge particles with a simultaneous increase in the volume of the charge, in which the swelling clay mass has encountered a universal unyielding resistance to swelling towards the end of the swelling process, resulting in a considerable build-up of pressure in the clay mass. However, the simultaneous swelling and sintering of the charge, which can be termed free foaming or free swelling, is thus unsuitable as a process on a commercial scale.
According to DE-OS 25 48 387, the supply of heat to produce the swelling could also be effected in the form of dielectric heating. As ceramic processes involving the supply of dialectric energy, in particular, are breaking new technological ground, the risk as regards process technology and apparatus technology in the case of this process is very high.
Processes for the manufacture of shaped bodies in a mold by means of heating occurring by combustion in the raw material are known (DE-AS 19 51 460 and DE 25 37 508). These processes have, however, the disadvantage that light building blocks of high porosity cannot be manufactured by means of these processes and that the quality and consequently the possibilities of use of the material thus created are severaly impaired on account of the unavoidable inclusions of combustion residues and the maintenance of temperature which is difficult to control. In order to reduce the combustion temperature to the necessary swelling temperature during the combustion in the raw material, combustion has to occur with a considerable excess of oxygen. A high oxygen content in the gases hinders the sintering process and the swelling, however, on account of too great a degree of incrustation. Moreover, the combustion in the raw material leads to the overall disturbance of the intergranular atmosphere.
DE-PS 19 14 372 described a process in which a universally unyielding, supported charge body, which is matched in its dimensions to the shaped body being manufactured, is initially formed from swellable granules of roughly uniform size, through which body a highly heated gas is then blown alternately from opposite sides for a short time until all granules have achieved a plastic bondable surface condition. What proves to be disadvantageous in the case of this process is the fact that adequate uniformity of the thermal treatment and heating speed of the product during the through-flow heating of a charge can only be obtained with an uneconomically high speed of flow, particularly because the closure of the interstitial volume of the charge which is dictated by the swelling makes a considerable rise of pressure necessary in order to maintain the flow. Furthermore, the gases flowing through the charge interfere with the development of an identical gas composition in the particles and between the particles, and influence the charge contents thermally and chemically, differently in their edge zones as compared with their core zone, especially with regard to reduction or oxidation of the particle shells, which leads to non-uniform product quality. The gases which are introduced from outside initially heat up the apparatus, which can lead to overheating of the apparatus and thus sticking of the material to the apparatus. Besides the lack of the possibility of uniform introduction of the gases into the raw material, associated with the shortness of the necessary treatment time, there occur considerable thermal and chemical variations in the condition of the gas along its path of flow in the raw material. The non-uniformity of temperature and chemical composition of the gases in the raw material which is so induced, causes localized variations in quality in the manufactured product, ranging from overburning to inadequate bonding of individual charge particles. It has been shown that the introduction of gases into the raw material for the purpose of heat supply to the raw material during the shaping by means of combustion in the raw material or by means of the flowing-through of heating gas is in fact capable of increasing the heating-up speed of the product considerably, but the speed is however still too low and, in particular, the treatment occurs to unevenly.
The processes constituting the prior art have considerable drawbacks with regard to thermal, chemical and mechanical treatment of the clay mass, with disadvantageous consequences in terms of expenditure on apparatus, energy expenditure, product quality and safety of the process, more especially in connection with heating, molding, supporting, caking and movement of the clay mass.
With regard to the heating of the clay mass, it can be confirmed that the clay mass is not heated sufficiently quickly, because the heat flow has to overcome too great a heat transportation resistance in the path in the clay mass or because heat on its way there is stored in other masses such as baking mold masses and is therefore not conveyed onto the clay mass, and because the energy current density or even the energy conversion density is not sufficiently high in the vicinity of the clay mass or the baking mold mass. Heat is stored in parts of the apparatus which are moved parallel to the clay mass, e.g. in rigid baking molds which are moved parallel to the clay mass or in crawler track links and belts, which, moreover, with regard to the energy expenditure, results in an increase of the heat energy costs on account of increased stored heat losses. The heat transportation resistance is too great if the heat transportation path in the clay mass is too long, e.g. because the clay mass has already been enlarged by cold foaming before the heating and the heat transportation path is extended by swelling not just during the heating, or if the heat transportation resistance around the clay mass is too great because it is enclosed by a rigid baking mold. Inadequate thermal treatment because of heating being too slow reduces the space-time yield and has the result that the apparatus needed is too large and consequently the apparatus expenditure and the energy expenditure are too high on account of too high heat energy costs resulting from too large wall heat losses. The energy costs dictated by the type of heating are too high because of too great gas heat losses through too large a quantity of exhaust gas or too high an exhaust gas temperature or through too high electrical energy costs resulting from heating by means of capacitive electrical heating with high conversion losses, this type of heating at high temperatures being additionally associated with a high innovation risk, or on account of a flow of the heating gas through the clay mass with great flow resistances.
With regard to the form of the clay mass used, it is established that the latter is molded by cold shaping either as a compact clay mass, or is divided, e.g. in the form of several individual partial clay masses which, for example, are combined into one charge body, or in the form of a clay mass with channels drawn therethrough or a cold-foamed porous clay mass.
DE-OS 28 14 315 proposed, for manufacturing a silicate material, a substance mixture which forms a foam at room temperature upon the addition of a further substance, which foam then has to be baked. If the clay mass has already been preformed in the cold shaping according to the external dimensions of the final shaped body, and if the clay mass is endowed with porosity, for example by being produced through cold-foaming or through blending with substances which burn out during baking, then the limit at which the baking can no longer be called baking with swelling but simple baking is reached, and hence is no longer relevant to the subject matter of the present application.
According to the prior art, the creation of a uniform shaped body by swelling of a compact clay mass is merely proposed in terms of using either a rigid bed or an air bed as a support below the swelling clay mass--a support in other directions is not mentioned.
The creation of a uniform shaped body by swelling of a divided clay mass can, according to the prior art, be termed conventional but disadvantageous.
If the mass is pre-swollen, and if therefore in the use of the clay mass being divided into individual partial clay masses, the swelling is carried out partly or exclusively before the necessary sintering of the partial clay masses in order to re-combine them into one whole clay mass, that is before the sintering of surfaces of the individual or connected partial clay masses, then the surfaces of the partial clay masses are oxidizes in order to stabilize them, to create a more solid shell and to make the surface non-adhesive. An attempt has been made to sinter the partial clay masses, which, as a result of the oxidation of their surfaces, have a reduced sintering capability, without pressure, resulting in a very small stiffness of the product, or by swelling pressure by means of further heating in a baking mold or by pressure exerted from externally, this resulting in slight uniformity through inner deformation during the pressing, in too high a shaping force expenditure and shaping energy expenditure and in too slight a stiffness of the product through inadequate sintering.
The supporting of the swelling clay mass is effected in a disadvantageous way both during swelling of a not pre-swollen clay mass and also during the swelling and sintering of a pre-swollen clay mass.
If the divided clay mass is so pre-swollen or so cold-formed that it already has the external dimensions of the shaped body constituting the product before the swelling or final swelling, then only the swelling which is necessary for closing the interstitial volume occurs, with simultaneous sintering, especially with all-round support of the clay mass if the clay mass is in the form of a charge, and the increase in volume takes place uniformly, since the greater part of the volume increase of the charge particles can be accomplished, while the mutual hindrance of them is prevented and the charge particles which are consequently regularly swollen are brought together in a prearranged uniform spatial density distribution in the dimensions of the block which is being produced, whereby no non-uniform increase in volume in order to close the interstitial volume is any longer even capable of substantially impairing the prearranged uniform spatial density distribution. During the swelling however, high pressure builds up in the clay mass on account of all-round unyielding support. With increasing pressure, the tendency of the swelling clay mass to cake on to the apparatus increases, in particular.
If the divided clay mass is not pre-swollen or is so cold-formed that, before swelling or final swelling, it has smaller external dimensions that the shaped body of the product, then the clay mass swells freely outwards, because there is not all-round support of the clay mass. The swelling thus occurs too unevenly, because the hot shape formation of the clay mass occurs in too uncontrolled a manner and at too low a pressure. Counter-pressure only at the end of the swelling, through an all-round unyielding rigid support for the subsequent equalization of the mass distribution within the shaped body volume is not possible to the extent necessary and is associated with too high a pressure between clay mass and apparatus.
The swelling of a divided and therefore not compact clay mass, not only during the swelling of a non-pre-swollen clay mass but also during the swelling and sintering of a pre-swollen clay mass into a large-size shaped body, occurs to feebly, particularly with invariable external dimensions of the clay mass, and occurs too unevenly, particularly with variable external dimensions of the clay mass, because the treatment, more especially the heating of a non-compact clay mass, occurs too slowly or too unevenly. If the heating of a non-compact clay mass is accomplished more rapidly, especially without any through-flow, so that less swelling capacity becomes lost, it occurs too unevenly, so that if there is no top wall or cover on the baking mold, there is a non-uniform density distribution in the shaped body or the shaped body has irregular external dimensions.
The caking of the clay mass on the apparatus which is guiding it is not prevented or is only preventable with too high an expenditure. An attempt has been made to prevent the caking of the clay mass on the apparatus resulting from the outer surface of the clay mass being reduced from the inside at high temperature during the swelling, by having separating means between the clay mass and the apparatus, such as graphite, sand, pressurized gas, or by using material for the apparatus which is supposed not to cake, such as magnesium oxide, magnesium chromite or similar, or by gas jet surfaces, or to render it harmless through shaping by means of permanent formwork, which results in too high an expenditure on the material of the apparatus and energy for gas jet flow or expenditure on raw materials, auxiliary means and operating means, or to prevent caking by keeping the surfaces of the apparatus which comes into contact with the swelling mass cold, and heating from inside the clay mass through heating by means of electrical heat generated inductively, capacitatively or conductively in the clay mass, which, however, requires high apparatus and energy costs to generate it.
The hot shaping occurs with too great a shaping force, and in consequence there is too great a bonding or frictional force arising from too high a pressure on the contact surface between clay mass and apparatus from inside or outside, and hence too high an essential power or energy expenditure for moving the clay mass.
The decisive disadvantages of the known processes are therefore to be seen on the one hand in the too slow or too uneven heating and also the too uneven chemical influence during the swelling process, with the consequence of high swelling gas loss and also too high a density and too uneven a density of the cellular ceramic shaped body and inadequate strength of the shaped body resulting from inadequate sintering in the case of charge particles which have to be sintered, and on the other hand in too high an energy consumption (through an insufficient degree of heat utilization in consequence of heat currents, more especially hot gas currents, which are inadequately directed from the point of view of process technology and plant technology), and also the danger of caking of the swelling clay mass on the apparatus and the inadequate shape stability of the soft clay mass during the swelling operation, and, in some cases, the sintering operation.
Summing up, it can be concluded that in the manufacturer of large-size porous ceramic shaped bodies of low density by means of heating and swelling of the clay mass, because of the necessary speed and uniformity of the heating during the swelling process, the danger of caking and the inadequate shape stability of the clay mass during the swelling, has, according to the prior art, especially if it is characterized by the formation of charges and the swelling of them and the necessity for sintering of the charge particles to each other, proved to be an insoluble problem, so that large-size shaped bodies with a uniformly low density and also uniform and high porosity, strength and precise shape have not hitherto been able to be manufactured economically.